Within one turn of the TCA cycle, substrate-level phosphorylation occurs at exactly one step. Which enzyme catalyzes this step, and what is the immediate high-energy product?

• A Fumarase, producing malate
• B Alpha-ketoglutarate dehydrogenase, producing NADH
• C Aconitase, producing citrate
• D Succinyl-CoA synthetase, producing GTP from GDP and Pi ✓

Explanation

Succinyl-CoA synthetase (succinate thiokinase) couples cleavage of the high-energy thioester bond of succinyl-CoA to phosphorylation of GDP, generating GTP, the only substrate-level phosphorylation in the cycle. Nucleoside diphosphate kinase can then transfer the phosphate to ADP. The other listed reactions conserve energy as reducing equivalents or rearrange carbon skeletons without producing a high-energy phosphate bond.

Reference: Lehninger Principles of Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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